1. Field of the Invention
The present invention relates in general to a control system for a hydraulic pump of the variable displacement type, and more particularly to a control system for a variable displacement hydraulic pump which is provided with a servo piston and a servo valve, including pressure responding pistons, which is connected to the servo piston by a feedback angle lever and the small diameter end of the servo piston is always applied with pump delivery pressure while the large diameter end of the servo piston is selectively applied with the pump delivery pressure in accordance with control of the servo valve.
2. Description of the Prior Art
Known control system for a hydraulic pump of the variable displacement type, such as disclosed in Japanese Patent Laid-open Publication No. Heisei. 1-116294, generally includes a servo valve and two control spool mechanisms, that is, a horsepower control spool mechanism and a pump delivery control spool mechanism for controlling output power and pump delivery of the hydraulic pump, respectively. The horsepower control mechanism is adapted to control a conduit, through which the pump delivery pressure or a servo pressure is supplied to the large diameter end of the servo piston of a servo cylinder, in accordance with variation of pump delivery pressure of the variable displacement pump applied thereto, while the pump delivery control mechanism is adapted to control a conduit, through which the pump delivery pressure is supplied to the large diameter end of the servo piston, in accordance with external pilot pressure along with the pump delivery pressure. In addition, this known control system includes three lever type of feedback lever mechanism comprising a feedback lever, adapted for connecting an end of a servo spool of the servo valve to the servo piston, and a pair of links adapted for selecting a displacement of the pilot spool mechanisms capable of causing the pump delivery to be reduced and causing the feedback lever to be operated in response to the selected displacement. Thanking for such a construction, the known control system controls the servo piston of the servo cylinder in accordance with variation of the pump delivery pressure or the external pilot pressure and, in this respect, controls the angle of inclination of swash plate of the hydraulic pump of the variable displacement type in order to control the pump delivery.
However, this known control system, including the horsepower control spool mechanism and the pump delivery control spool mechanism, requires the servo valve, a pair of spools responding to the pressures, the three lever type of feedback lever comprising the feedback lever and the pair of links, in result, the construction of the system is inevitably complicated. Also, in this known control system, the horsepower control spool mechanism and the pump delivery control spool mechanism are separately operated so that the known control system has a problem in that the pump delivery control range according to the external pilot pressure, when the pump delivery pressure and the external pilot pressure are applied to the mechanisms at the same time, is narrow under the condition of high pump delivery pressure as shown in the graph of FIG. 8b and this deteriorates the control performance in controlling the pump delivery. Furthermore, the known control system is provided with at least two compression coil springs for biasing the spools of the spool mechanisms to a direction opposite to a biasing direction of the actuating force generated by the pump delivery pressure and the external pilot pressure applied to the spools, respectively. In this regard, the relation between the pump delivery Q and the pump delivery pressure Pd has the characteristic curve comprising the three straight lines having different gradients and continued at two inflection points as shown In FIG. 7b. Thus, the known control system has another problem in that the output power of an engine for driving the hydraulic pump is not optimally utilized at about the two inflection points of the characteristic curve.